1. Field of the Invention
The present invention relates to a fixing device for an image forming apparatus, and more particularly to the fixing device that provides desired image glossiness according to types of images.
2. Discussion of the Background
In an image forming apparatus, such as a copying machine, a facsimile machine, and a printer, a printed (copied) sheet is obtained by fixing an unfixed image onto a sheet as a recording material by heat and pressure in a fixing device.
The fixing device may employ any one of fixing methods such as xe2x80x9cheating roller fixing methodxe2x80x9d, xe2x80x9cradiant/flash fixing methodxe2x80x9d, and xe2x80x9coven fixing methodxe2x80x9d. Specifically, in the heating roller fixing method, a sheet carrying an unfixed image is conveyed while being sandwiched between a pair of rollers including a heating roller, and the unfixed image is fixed onto the sheet by heat of the heating roller. In the radiant/flash fixing method, a fixing device includes an infrared/Xenon lamp as a heat source, and an unfixed image is fixed onto a sheet by radiant heat. In the oven fixing method, heating elements are arranged in the vicinity of both sides of a sheet, and an unfixed image is fixed onto the sheet by convected heat.
In the above-described fixing methods, the heating roller fixing method has been often employed because of advantages such as high thermal efficiency, high printing speed, high efficiency of heat transfer, and stabilized fixing efficiency. Further, a construction of an image forming apparatus including a fixing device using the heating roller fixing method may be simple, because the fixing device may also serve as a sheet conveying device.
In the heating roller fixing method, when toner on a sheet is changed from a solid to liquid state by heat and pressure, the toner is changed from a partly fused and coalescent condition to a fluid and spread condition, and is finally in a penetrating condition on the sheet. As a result, particulate toner of an image, which has been likely to be removed from a sheet, is fused and fixed onto the sheet.
When a toner image is fixed onto a sheet, fixing efficiency depends on the conditions such as temperature, heating time, and contact pressure. Therefore, respective conditions are needed to be set according to types of images.
In a case of multi-color image forming apparatus that forms multi-color images, image glossiness may be changed according to desired types of images. For example, when compared to photographic image and character image, glossiness of the photographic image, which depends on an amount of reflected light relative to that of incident light, is desired to be enhanced because of demand for high quality image.
As a background method of obtaining image glossiness according to types of images, Japanese Laid-open Patent Publications No. 63-300254 and No. 4-204669 respectively describe image forming apparatuses in which the glossiness of a black character image is decreased by increasing the viscosity and coefficient of elasticity of black toner more than those of other color toner.
Further, Japanese Laid-open Patent Publications No. 10-123795, No. 11-24463, and No. 5-333643 respectively describe image forming apparatuses in which the glossiness of a multi-color image is increased by changing the above-described temperature and contact time.
In the case of the background method of controlling the glossiness of a black character image, even though such the method is applied to a multi-color image, desired image glossiness may not be obtained.
In the case of the background method of controlling the glossiness of a multi-color image, desired glossiness of the multi-color image may be selectable, but the following concerns may be raised.
First, when temperature is switched between a multi-color mode and a single-color mode, because temperature may not be adjusted quickly, time for forming images may be increased. Further, in the case of low temperature, so-called cold offset condition (i.e., a part of toner image is adhered to a fixing member of a fixing device) and insufficient coloring may be likely to occur. In the case of high temperature, so-called hot offset condition (i.e., a part of toner image is adhered to a fixing member of a fixing device) winding of a sheet around a roller may occur. Accordingly, a range of switches of temperature results is narrowed. If the temperature is switched within a range in which the above-described defective conditions are avoided, desired image glossiness may not be obtained.
Second, when linear velocity of a fixing member is changed, it takes time for a sheet to be conveyed depending on the linear velocity. As a result, productivity at the time of image formation may be decreased.
Third, when changing pressure force of a fixing member, it may affect a sheet conveyance depending on flexural rigidity of a sheet. Particularly, when a thin sheet is used, wrinkles on the thin sheet and sheet jam may occur.
Another method of controlling image glossiness, which addresses the above-described concerns, is proposed. Specifically, a fixing device includes first and second fixing units respectively having pairs of heating and pressure members along a sheet conveying path. In such a fixing device, image glossiness is changed by making the surface roughness of one of the heating and pressure members of the second fixing unit which contacts a toner image on a sheet different from that of one of the heating and pressure members of the first fixing unit which contacts the toner image on the sheet.
In the above-described fixing device, control of temperature of toner of a toner image on a sheet is required when the surface of the toner of the toner image on the sheet is pressed by the pressure member of the second fixing unit arranged at a position away from the first fixing unit. Because a construction for controlling temperature and a heating construction are necessary for the second fixing unit in addition to the first fixing unit, a number of construction parts may be increased, thereby increasing the cost of an apparatus.
According to one aspect of the present invention, a fixing device that fixes an unfixed toner image onto a moving sheet, includes a first fixing unit configured to make toner of the unfixed toner image penetrate into the sheet by heat and pressure so as to be fixed onto the sheet as a fixed toner image, and a second fixing unit arranged downstream of the first fixing unit in a sheet moving direction and configured to contact and press the fixed toner image on the sheet to change smoothness of a surface of the toner of the fixed toner image on the sheet. The second fixing unit is arranged at a position where a temperature of the toner of the fixed toner image on the sheet is greater than a glass transition temperature.
According to another aspect of the present invention, a fixing device that fixes an unfixed toner image onto a moving sheet, includes a first fixing unit configured to make toner of the unfixed toner image penetrate into the sheet by heat and pressure so as to be fixed onto the sheet as a fixed toner image, a second fixing unit arranged downstream of the first fixing unit in a sheet moving direction and configured to contact and press the fixed toner image on the sheet to change smoothness of a surface of the toner of the fixed toner image on the sheet, and a first movable belt provided to span the first and second fixing units along the sheet moving direction and configured to move along the sheet moving direction and to contact a surface of the toner of the fixed toner image on the sheet between the first and the second fixing units. The first belt includes a thermal storage property.
Objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.